What is the difference between 関係（かんけい）and 関連（かんれん）?
It must be difficult to answer, even in Japanese!
First of all, please have a look at both Kanji carefully.
The second Kanjis are「係」and 「連」.
In Japanese 「かかり：かかわる：」and「れん：つらなる」
So, it supposed be 関係 means かかわる+かかわる、関連 means かかわる+つらなる。
関係 has wider range of meaning than 関連.
関係 means some relation between more than two things, or influenced from the other .
On the other hand, 関連 also express some connection between more than two matters, but in this case it has to be connected as contents of the matter.
（The police is investigating the connection/relation between two cases which happened last week.)
Both 関係 and 関連 can be used.
He seems that he is related to this case.
I hope it will help you.